The background of the invention will be discussed in two parts.
1. Field of the Invention
This invention relates to differential pressure responsive devices, and more particularly to a spring assembly for differential pressure responsive devices which is readily adjustable to different spring tension ranges.
2. Description of the Prior Art
The use of more than one spring to accommodate varying loads has been employed in numerous applications, such as shock absorption on railroad cars or the like. In such applications, helical springs are employed, in nested or coaxial arrangement, with spring lengths within a nested arrangement varied to provide coaction of subsequent springs after certain loads are reached.
One such railroad vehicle suspension assembly is shown and described in U.S. Pat. No. 4,148,469, entitled "Dual Rate Spring with Elastic Spring Coupling", issued Apr. 10, 1979, to Geyer, which discloses a supportive spring system for accommodating variable loads of railway cars, the spring system utilizing a plurality of helical spring units comprising multiple nested springs with primary springs having greater axial length than one or more secondary springs so that the primary springs alone provide the initial load support, with the secondary springs becoming supportive under additional load, with all springs thereafter acting in unison.
Another spring device particularly adapted for use in a suspension system of a railroad car truck is shown and described in U.S. Pat. No. 4,186,914, entitled "Dual Rate Spring Device for Railroad Car Trucks", issued Feb. 5, 1980 to Radwill et al. The device includes a vertically mounted light duty spring means positioned between a side frame of the truck and a load carrying bolster of the truck to support the bolster when in a lightly loaded condition. Positioned around the light duty spring means is a shorter heavy duty spring which with the light duty spring supports the bolster when in a heavily loaded condition.
The use of nested helical springs of different lengths has also found application in other mechanical devices, such as compressors. One such structure is shown and described in U.S. Pat. No. 3,862,751, entitled "Dual Stage Compressor Spring", issued Jan. 28, 1975, to Schwaller, and discloses a dual stage energy absorbing spring assembly for use in compressors, the spring assemblies comprising two concentrically disposed, oppositely helical springs which are provided with buttons at top and bottom to provide bearing surface, with one spring being longer than the other such that as the assembly is compressed first one, then two springs resist compression, thus increasing spring rate when the second spring is engaged.
Another application of use of a variable spring tension arrangement is shown and described in U.S. Pat. No. 2,599,403, entitled "Automatic Spring Tension Varying Means", issued June 3, 1952, to D. Martin, and discloses a device for use in slot machines to alter the effective tension of the spinner-actuating spring element as an automatic reaction to play of the machine by use of a second spring acting in concert with a primary spring, with the second spring connected to an eccentric member which enables varying the force on play of the machine.
In electromechanical instrumentation and control devices, bellows and spring assemblies are employed to actuate levers or linkages to control dials or the like to provide a visual indication of a parameter, such as pressure. In flow or pressure instrumentation devices, in particular, to provide for different ranges within a given instrument, and to achieve accuracy within th range, normally a different spring assembly is incorporated in the instrument suitable to the range, thus requiring that different spring range subassemblies be available on the production line for the different ranges needed.
A typical bellows and spring arrangement in a differential pressure responsive device is shown and described in U.S. Pat. No. 4,445,382, entitled "Differential Pressure-Responsive Device", issued May 1, 1984, to Reynolds et al, such device including a bellows, the movement of which is responsive to pressure, with a helical range spring having one end thereof affixed to the bellows, with the other end being restrained by a plate member, the bellows and range spring being assembled as part of a readily removable module which also includes the high and low pressure relief valve assemblies.
Two patents which issued to C. B. Nolte disclose bellows and range spring structures similar to the Reynolds et al device. These devices are disclosed in U.S. Pat. No. 3,422,680, entitled "Differential Pressure-Responsive Device", issued Jan. 21, 1969, and U.S. Pat. No. 3,718,048, entitled "Relief Valves for Differential Pressure Flow Meters", issued Feb. 27, 1973. However, in the U.S. Pat. No. 3,422,680 Nolte patent, in lieu of imposing a positional restraint on the range spring at the end opposite the bellows, the device includes a dual spiral flat spring member, which engages the upper end of the range springs.
A variable spring assembly is disclosed in U.S. Pat. No. 4,112,960, entitled "Variable-Gradient Spring Sub-Assembly", issued Sept. 12, 1978 to Hermanns, the range spring sub-assembly having a helical spring whose lower end is secured to the pivoted feedback arm of the feedback assembly. The upper end is coupled by a span adjustment screw to a fixed arm thereof. The screw is coupled to a cylindrical coupling head which is provided with projections which engage the space between adjacent coils of the spring, with turning of the screw varying the position of the coupling head relative to the convolutions of the helical spring, thereby varying the number of active convolutions of the spring, to thereby vary the gradient of the spring.
U.S. Pat. No. 2,620,413, entitled "Control Device", issued Dec. 2, 1952, to B. L. Johnson, and discloses a control device in which a snap action switch is actuated in response to changes in variable condition and having two coaxially mounted springs in operative relation with the switch actuating lever, and wherein the tension of at least one of the springs may be adjusted by a thumbscrew in coaction therewith.
In accordance with an aspect of the invention, it is an object to provide a new and improved range spring assembly for differential pressure responsive devices, which assembly may be readily adjusted for different ranges.